The present invention generally relates to spread spectrum communications, and more particularly to a spread spectrum communications system which uses a spread spectrum signal multiplied with a pseudo noise code sequence for transmitting and receiving a message. This system is applicable to wireless communications such as radio transmission systems, and enables multiple access, increased immunity to noise, and increased resistance to interference.
Generally, a spread spectrum communications system makes use of a sequential noise-like signal structure, for example, pseudo noise codes (hereinafter called pn codes), to spread a normally narrowband information signal over a relatively wide band of frequencies. At the transmitter of the spread spectrum communications system, a spread spectrum (ss) signal in which a carrier multiplied with a pn sequence is modulated in accordance with the information signal is transmitted. The receiver of the system correlates the signals to retrieve the original information. The basic signal characteristics of the spread spectrum communications system include: the carrier of the ss signal is an unpredictable, wideband signal multiplied with a pn code sequence, the bandwidth of the carrier is much wider than the bandwidth of the data modulation; and demodulation is accomplished by correlation of the received wideband signal with a synchronously generated replica of the wideband signal The advantages of the ss communications system include the unpredictability of the wideband signal, the security of the transmitted data signal, and the multiple access capability.
In the prior art, there is a spread spectrum method for data transmission which uses optical transmission for spread spectrum communications. This method is disclosed in "A Study of Optical Transmission in Space Using Spread Spectrum Method" by M. Nakazono et al., a transaction of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan, pp.39-43, SSTA89-17, August, 1989. In the above mentioned spread spectrum communications method, a spread spectrum (ss) signal in which information is multiplied with pn codes is produced and the ss signal is transmitted by using light as the carrier. If this optical method is applied to a spread spectrum communications system using a radio wave as the carrier, the spread of frequencies of the ss signal is not enough to secure the information, and the spectrum of the ss signal contains the carrier frequency as a very large component of the spectrum.
There is another spread spectrum communications method in the prior art. This method is disclosed in "A Method of Fast Data Transmission For A Spread Spectrum Modem Using a SAW Convolver" by Y. Uchida et al., a transaction of the IEICE of Japan pp.105-111, SSTA90-32, June, 1990. In this spread spectrum communications method, a spread spectrum (ss) signal in which a carrier is switched on and off in accordance with a pn code sequence is modulated in accordance with an information signal, and the thus generated ss signal is transmitted. However, when the ss signal is received at any external receiver of a third party, the received ss signal can easily be demodulated to reproduce the original information with no need of using the pn code sequence because the spectrum thereof contains a very large carrier component. Thus, the security of the information is not available with the above conventional system.